Constant resistivity alloy



ganin.

Patented Feb. 1,1933

UNITED STATES PATENT OFFICE IICHAEL G. COB-SON, OE JACKSON HEIGHTS, NEW YORK, ASSIGNOR TO WESTON ELEC- TBICAL INSTRUMENT CORPORATION, OF NEWARK, NEW JERSEY, A CORPORATION OF NEW JERSEY Ho Drawing.

This invention relates to alloys which exhibit a substantially constant electrical resistivity over a wide range of temperature,

and particularly to alloys of the copper base yp I Objects of the invention are to provide alloys of the copper-manganese type which, as compared with known com ositions, show a marked improvement in t e constancy of their electrical resistance. A further object methods of preparin such alloys to improve the uniformity of t eir temperature-resistance characteristics.

It is well recognized that certain coppermanganese alloys exhibit an approximately constant electrical resistivity over a wide range of temperature, and such alloys are sold commercially under the name Man- My investigations on the nature and properties of manganin have led me to believe that the peculiar temperature-resistance characteristics of such copper base alloys are due to the combination of two influences. In one respect, these alloys behave substantially like other copper base alloys for which the product of electrical resistivity by the temperature factor oscillates closely around a constant figre. On the otherhand, a certain process is always taking place when these alloys are subjected to temperature changes, which process may be described, in a hypothetical wa as a formation of and dissociation of ad itional bonds between the atoms of copper and manganese. This proccous'rm nnsrs'rrvrriz- ALLOY Application filed October 14, 1980. Serial No. 488,697.

out of the formed compound as a separate phase.

An increase in the dissociation of this compound, like the ionic dissociation of a solute Within a solvent, tends to increase-the conductivity of the whole system and consequently it counteracts the increase in resistivity due to the increase in temperature.

The constant of the chemical process involved increases with the temperature as a function of a higher order and for that reason the increase. in conductivity "due to temperature increase becomes ever stronger. As long as the alloy is in the domain of temperatures much lower than the normal room temperature, the second process does not completely balance the natural increase in resistivity and the alloy has a positive temperature faccomes more positive until at very high temperatures the alloy behaves again in a nor-' mal way. I

This feature is encountered in practically every composition in the copper manganese series but the total amount of the compound present and the rate of its decomposition with the increase in temperature depend u on the amount of manganese present. In al oys low in manganese the natural temperature coeflicient is high and its decrease due to the chemical process involved so small that a curve representing the actual change of resistivity withtemperature shows only a strong bend. With the increase in the content. of manganese this bend becomes more pro nounced, and the tendency for it to go through a maximum and'to become negative beyond it increases. The more manganese is present,

the closer the maximum approaches the normal temperatures and alloys containing from tor. Alloy samples characterized by such curves cannot be regarded as reliable for the construction of instruments of precision.

It is doubtful whether this feature was ever described before but in an empirical way it was counteracted to an extent by adding to the alloy another element which stays in solid solution in copper and cannot be easily burned away. Such element as usually encountered is nickel. The regular manganin of Com-.

merce contains about 12% manganese and 4% nickel, the balance being copper.

The addition of the nickel gives rise to another eifect. It evidently changes the dissociation constant of the ,copper manganese compound present; in fact, it increases the dissociation constant. For that reason these commercial manganins have a range of high constancy which, as compared with the binary alloys, is shifted down very closely to normal or atmospheric temperature. On the other hand, the increase in dissociation beyond normal temperature becomes so strong that even the best manganin of commerce shows about 2% of change in the negative direction when approaching 100 (.1.

In accordance with the present invention the temperature-resistance characteristics of copper-manganese alloys are stabilized and improved by the addition of an element or elements which do not give rise to the undesiredeifects of nickel.

I have discovered that aluminum, either alone or in association with a smaller quantity of iron, is far superior to nickel as an additional element inalloys of the copper-mam ganese type. Aluminum decreases the natural temperature coeificient of the alloys consideraby more than nickel does, and therefore makes it easier to balance the natural positive temperature coefficient of the alloy by the relatively small dissociation which takes place at low temperatures. Like nickel, aluminum increases the dissociation at higher temperatures, but to a somewhat lower degree. Furthermore, the aluminum prevents the formation, even in an oxidized alloy, of an outer layer of such high conductivity as ob- 'tains in binary copper-manganese alloys and,

to a lesser extent, in manganin containing nickel.

For best results, smaller amounts of manganese can and should be used when aluminum 1s presen As compared with the usual 12 to 14% of manganese which has been used in a commercial manganin, the manganese may be reduced to from 7 to 11% when aluminum is present to the extent of from 3 to 4.5%. A further flattening of the temperature-resistivity curve may be effected by the addition of from .2 to 1% of iron, although for many uses this further improvement in constancy may not be important.

One particular alloy which exhibits very high constancy of electrical resistivity consists of 9.5% manganese, 3.5% aluminum, 0.5% iron, and the balance copper.

A further advantage accompanying the use of aluminum in copper base alloys is the lowering of the thermo-electric voltage developed between copper and the alloy. The potential developed between a commercial manganin and copper may run from about 0.7 to 1.5 microvolts per degree, while an alloy including, for example, 9% man anese and 4% aluminum will develop only mm 0.1 to 0.5 microvolts.

I have also found that the addition of tin to the aluminium-containing alloys is advantageous, particularly in ameliorating other physical properties of the alloys. For instance, the capacity of the alloy for taking solder is advantageously afiected by the addition of tin. An alloy of the following composition has been found, for example, to have very desirable characteristics:

Per cent Manganese 10 .Aluminum 3 Tin 2 the balance being copper.

Although the addition of aluminum reduces those variations in resistivity which arise from surface oxidation of the alloy, I have found that the resistivity curves of specimens of like composition will coincide when the annealing is carried out under nonoxidizing conditions, such as obtained by a high vacuum. Except in thecase of thin bars, strips or wires, the preliminary annealing operations may be performed in air or under charcoal, if the specimens are then carefully pickled. The last anneal should be carried out, however, under high vacuum or equivalent non-oxidizing conditions. Temperatures of from 500-550- C. are appropriate for the annealing of the copper base alloys.

Alloys such as contemplated 'by this invention may, of course, be made up in the usual manner in the form of bars, ribbons, strands or wire.

DOT

WhatIclaimis:

1. An alloy of constant resistivity type comprising copper, manganese 7 to 11% and .{aluminum 3 to 4.5%.

" 2. An alloy of constant resistivity type comprising copper, manganese 7 to 11%, aluminum 3 to 4.5% and iron 0.2 to 1%.

3. An alloy of constant resistivity type mi .consisting of copper, manganese 7 to 11%,

aluminum 3 to 4.5% and tin about 2%;

4. A constant electrical resistivity alloy acomprising 9.5% manganese, 3.5% alumlmum, and the balance co per.

5. A constant electrical resistivity alloy comprising 9.5% man anese, 3.5% aluminum, 0.5% iron and the alance co 1'.

6. A constant electrical resistivity alloy comprising 10% manganese, 3% aluminum, 2% tin and the balance copper.

In testimony whereof, I afiix m signature.

MICHAEL G. RSON. 

